What goes wrong in a chimney nobody uses

An idle chimney is still outdoors

The problems with an unused chimney begin with a simple fact: stopping fires stops one kind of wear and leaves the rest in place. Crown, flashing, cap and the brick above the roof line take the same sun and rain whether the fireplace is used every week or not at all. What changes is that nobody looks up the flue, so problems that a yearly sweep would have caught go unnoticed.

An unused flue also stays cool and damp. With no fires to warm and dry it, water that gets in through an open top or a cracked crown has more time to soak the masonry, rust the damper and feed the musty smell an idle fireplace can develop.

Air moving the wrong way

A chimney is a hole through the house to the sky, and air moves through it whether or not there is a fire. When bath and kitchen fans, a clothes dryer or the air handler pull the house slightly negative, air can be drawn down the flue and into the room. It brings with it the smell of old creosote and damp masonry, and so an idle fireplace can smell worst on days when nothing has changed inside it.

The flow can reverse too. With the damper open or a throat damper that no longer seals, warm or cooled indoor air can rise out through the flue. If the house is air-conditioned, that is conditioned air leaving through the chimney instead of staying in the room.

Animals move into quiet chimneys

A tall, dark, sheltered shaft with no smoke in it is attractive to wildlife. Birds nest in it, and bats and other animals can roost or den in it. Some of these animals are protected, and chimney swifts and bats in particular come with rules on when they can be excluded and when a chimney can be capped. A screened cap on an empty flue prevents the problem, and it is far easier than dealing with a nest after the fact.

Is it really unused?

Before closing anything off, confirm what the chimney serves. A single chimney can hold several flues, and one of them may still vent a furnace, a boiler or a water heater even though the fireplace flue beside it has been cold for years. A flue that serves an appliance has to stay open to the outdoors and sized for that appliance, and it cannot be capped shut or plugged.

Decide, too, whether the fireplace is retired for good or just resting. A fireplace you may use again is treated differently from one that is finished with fire for good.

Four ways to close it off

Each option trades simplicity against how tightly it closes the chimney. They are listed from lightest to most permanent, so the simplest way to cap off an unused chimney comes first.

• A screened cap: stops rain, debris and animals at the top and leaves the flue open to air. It is the simplest option and suits a chimney that may be used again.
• A top-sealing damper: a gasketed lid at the top of the flue, closed with a cable from the firebox. It shuts out air, rain and animals, and it opens again whenever a fire is wanted.
• Sealing the flue: a solid cover at the top and the fireplace opening closed off below. The method has to let the masonry dry, since a chimney sealed airtight at both ends can hold moisture, and the fireplace should be marked so nobody lights a fire in it.
• Removal: dismantling the chimney, either to a point under the roof or all the way down, which is a construction project involving roofing and framing as well as masonry, and a much bigger decision than the other three.

How to close off a fireplace from the room side

Whatever is done at the top, the fireplace opening is where the room meets the chimney. A throat damper that still closes well is the first line, and a damper that is rusted open or warped leaves the room connected to the flue. Covers made to fit the fireplace opening, and draft stoppers set in the throat, add a seal from below.

Anything placed in the opening or the throat has one rule: it comes out before a fire, every time. A card or tag at the fireplace saying what is installed keeps a guest or a new owner from lighting a fire against a plug they did not know about.

Picking between them

How the fireplace will be used decides it. A cap is the right answer when the goal is simply to keep weather and animals out. A top-sealing damper earns its place when air moving through the flue, whether drafts, odors or lost conditioned air, is the complaint, or when the throat damper has failed. Sealing makes sense for a fireplace that is truly retired, and removal is a separate conversation about the house itself.

Whatever is chosen, the parts that keep water out still need to be sound. Putting a cap or damper on a chimney whose crown is cracked or whose flashing has failed leaves the main leak in place. Our chimney cap replacement page covers sizing and fitting a cap.

Keeping an idle chimney in shape

Even a chimney that will not be used needs an occasional look at the crown, flashing, cap and upper brick, since those are what keep water out of the house. A check at the same yearly interval as a working chimney is a sensible habit, and it catches cracks and rust while they are small.

If you decide to use the fireplace again after years of rest, have it swept and inspected before the first fire. Old deposits, nests, fallen debris and water damage can all be waiting in a flue that has sat idle, and none of them are visible from the room.

If the house changes hands, tell the buyer what was done to the chimney and why. A sealed flue, a plug in the throat or a damper wired shut is exactly the kind of detail a new owner needs before lighting a first fire.